Murine committed hemopoietic stem cells (CFU-C) can proliferate and differentiate to mature granulocytes and macrophages in the presence of an external source of the lymphokine/monokine colony stimulating factor (CSF). In the present study we investigated whether BM cells can generate their own CSF. BM cells can be induced to generate CSF by simultaneously incubating with the tumor promoting agent 12-0-tetradecanoyl-phorbol-13-ester-acetate (TPA) and bacterial lipopolysaccharide (LPS). Quantitation of CSF was done by measuring 3H-thymidine incorporation into the DNA of a basophil/mast cell line (PT-18) which is dependent on CSF. Replacing TPA with its non tumor promoter 4-0-Methyl TPA or treating BM cells from C3H/HeJ mice (low responder to LPS) with TPA and LPS did not generate and CSF. Lipid A but not the polysaccharide moiety could replace LPS in inducing with TPA generation of CSF. Incubation of BM cells with TPA and LPS for 2-4 hr was sufficient to induce generation of CSF followed by an additional incubation of 20 hr without TPA and LPS. Cyclohexemide inhibited the generation of CSF. IL-1 could not replace either TPA or LPS in inducing generation of CSF. To determine whether TPA and LPS affected different cell populations leading to CSF generation, BM cells were separated into 2 aliquots, one treated with TPA and the other with LPS. After 4 hr the cells were washed and the 2 aliquots recombined and incubated for an additional 20 hr. No CSF was generated.